CVS S9 - Incomplete Special Circulations

Question 1

Describe how the pulmonary circulation is specialised to cope with the entire cardiac output

Answer 1

Has very low resistance due to short, wide vessels with lots of capillaries in parallel
Arterioles have relatively little smooth muscle which also reduces resistance

Question 2

What is the normal pressure in the pulmonary artery?

Answer 2

12-15mmHg

Question 3

What is the normal pressure in the pulmonary capillaries?

Answer 3

9-12mmHg

Question 4

What is the normal pressure in the pulmonary veins?

Answer 4

5mmHg

Question 5

What is the optimal ventilation/perfusion ratio?

Answer 5

0.8

Question 6

What is hypoxic pulmonary vasoconstriction?

Answer 6

Alveolar hypoxia results in the vasoconstriction of pulmonary vessels
This increases resistance so causes decreased flow to less well ventilated areas and greater flow to the better ventilated alveoli
This helps maintain the ventilation/perfusion ratio of about 0.8

Question 7

When can hypoxic pulmonary vasoconstriction be a problem?

Answer 7

In cases of chronic hypoxic vasoconstriction

This can cause right ventricular hypertrophy and heart failure due to increase in after load

Question 8

Describe the forces which determine the formation of tissue fluid in the lung

Answer 8

Hydrostatic pressure of the blood within the capillary forces blood out
Colloid osmotic (aka oncotic) pressure exerted by large molecules in blood vessels draws fluid back in
As the pulmonary capillaries are low pressure, little fluid leaves the vessels but if pressure increases, oedema can occur
When sitting upright, gravity places greater hydrostatic pressure on the base of the lung

Question 9

Describe the relationship between the mechanical work and oxygen demand of the myocardium

Answer 9

There is an almost linear relationship between the work done and the oxygen demand of the myocardium (as work increases, so does O2 demand)

Question 10

Describe the coronary circulation

Answer 10

Left and right coronary arteries arise from the left and right aortic sinuses
These fill in diastole
There is a high capillary density in the myocardium to efficiently deliver oxygen
NO is constantly produced by the coronary endothelium to maintain high basal flow

Question 11

Describe the blood supply to the lungs

Answer 11

Two circulations: pulmonary and bronchial
The bronchial is part of the systemic circulation and fulfils the lungs metabolic requirements
The pulmonary circulation is to the alveoli and is for gas exchange